Heat staking is a process for permanently joining first and second parts at one or more discrete points. One of the parts to be joined includes a short plastic post or stud which is inserted through a hole or aperture of roughly equivalent size in other part. The plastic stud is then heated until it is plastically deformable and the end of the stud is flattened with a metal punch to form a rivet-like head which locks the two parts together.
It is possible to accomplish the heating and the flattening of the stud simultaneously by heating the punch prior to bringing into contact with the stud, the punch transferring its heat to the stud to soften it is being shaped. In such an operation, the punch is typically resistance heated by applying electrical current to the punch. Heat staking machines operating in this manner are disclosed in U.S. Pat. No. 4,767,298 and U.S. Pat. No. 5,227,173.
Another known heat staking technique is to heat the stud prior to it being contacted by the punch. In the past, this has been achieved by blowing hot air over the stud. U.S. Pat. No. 5,018,957 discloses a staking machine using electric heaters to generate the hot air and blowers to circulate the hot air over the stud. In some manufacturing operations, this pre-impact heating of the stud has been found to be advantageous in that it minimizes the amount of residual stress in the deformed stud after it has cooled. In the past, however, the apparatus necessary for the heating and circulation of air has resulted in a relatively large and mechanically complicated machine. Also, such a machine is relatively energy inefficient in that a large percentage of the heat generated to is not transferred to the stud but rather is wasted.
It is therefore desirable to provide a heat staking machine that is energy efficient and that is simple and compact in construction.